Stretchable electronic components have applications in flexible electronics, biomedical devices, and soft robotics. Room-temperature liquid metals may be attractive materials for fabrication of such devices because they retain their functionality even when stretched to several times their original length. One of the earliest examples of liquid-phase electronics is the Whitney strain gauge. This device measures strain of a mercury-filled rubber tube by measuring change in electric resistance of the metal. While in the past two decades mercury and rubber have been replaced by nontoxic liquid gallium alloys (i.e., GaIn and GaInSn) and more elastic polydiemethylsiloxane (“PDMS”), the resistive design of the liquid metal strain sensor remains popular. These resistive devices have a large footprint that may restrict the number of sensors that may be embedded into, for example, electronic fabric or skin. For example a ˜1Ω resistor made out of GaInSn with resistivity of 0.29 μΩm in a 200 μm diameter channel typically has a length of ˜10 cm. By winding the channel 10 times such a sensor may fit into an area of ˜1 cm2.
Improved understanding of the GaIn and GaInSn wetting characteristics and advances in their micro-fabrication may enable fabrication of smaller liquid metal filled microchannels with higher areal density; however, the serpentine geometry of these resistors remains quite complex. Several designs of capacitive strain sensors have been proposed as alternatives to the resistive devices. These capacitive devices consist of two microchannels filled with liquid metal separated by solid dielectric PDMS matrix. For in-plane sensing an order of magnitude estimate for the required sensor footprint can be obtained using the parallel plate capacitor model, C≈ε0εA/d (i.e., A≈Cd/ε0ε). To achieve a capacitance (C) of ˜1 pF, two liquid metal-filled microchannels with both height (h) and separation (d) of ˜400 μm within a PDMS matrix (ε˜2) typically have a length of 1˜10 cm (from the conductor-dielectric interfacial area, A˜1 h˜4×10−5 m2). By winding the parallel channels in a serpentine arrangement, such a sensor may fit into a base area of several square centimeters. Such a base area is typically needed for a variety of winding two-channel capacitive strain sensor designs to achieve C˜1-15 pF. With such a large footprint the sensor output is affected by stretching in multiple directions, not only in the desired principle direction. As a result, correlation of the physical strain with the sensor output is complex.